(Not Applicable)
Any type of rotating machinery subjects operating personnel to danger from flying projectiles. High speed centrifuges are particularly dangerous since their purpose is to spin test objects, usually at the end of a rotating arm. If something should fail, either the test object or part of the rotating arm can leave the centrifuge tangentially with a high kinetic energy.
For safety purposes, most centrifuges are designed with some sort of primary containment or enclosure surrounding the rotating components of the machine to contain any form of fragment that might be released from the machine. However, there are always occasions when the primary containment is opened, such as for loading test parts, temperature conditioning, or machine maintenance. If these entry points are not closed and interlocked properly, or if the interlocks are purposely bypassed, or if the machine would fail in a mode not completely understood by the designers, a catastrophic failure of the primary containment can occur, causing part fragments to be thrown from the machine with a high kinetic energy.
In addition, other types of rotating machinery also have the capability of ejecting projectiles with high kinetic energy.
Any failure mode of a rotating machine can cause damage to equipment in the vicinity and also personnel injury or death. These failure modes are common to all centrifuges and any high speed rotating machinery. Because of a history of machine failures and uncertain methods in determining failure criteria, it is considered a good safety practice to surround all rotating machinery, not only with primary containment, but with a secondary method of containment to protect operating personnel. The secondary containment should be designed to contain any fragments that could breach the primary containment.
Typical barrier designs which could be utilized for secondary containment of a centrifuge include steel plate/frame structures, reinforced concrete walls, concrete block walls, and a concrete walled excavated pit with either a steel or concrete roof. These classical machine barrier designs can be quite expensive to fabricate and install. An idea for a innovative and economical approach to machine containment was needed.
Polycarbonate sheets (such as Lexan(copyright)) have long been used as transparent barriers. For example, the Oklahoma County Detention Center uses Lexan sheets bolted to a Unistrute(copyright) frame (Unistrut Corp., Wayne, Mich.) to enclose a mezzanine within the institution. Of course, this application does not anticipate the containment of a centrifuge or similar equipment, nor is it modular and easily assembled and moved.
There has been limited testing of polycarbonate used in a shielding application. Most of the data available is for the ballistic regime at velocities of 1000-3000 ft/s. Most of the research performed on characterizing polycarbonate began in the 1960""s and was classified and therefore not included in this report. The U.S. Air Force performed a minor amount of ballistic testing to determine the material""s applicability to transparent armor. The research also included early bonding agents to bond polycarbonate with glass. One of the most significant conclusions from this early work was the determination that the ballistic performance of polycarbonate is not related to its low-rate impact properties. This fact is significant in that there has not been a great amount of testing performed on low-rate impacts. The U.S. Army Ballistic Research Laboratory investigated the impact resistance of various glazing materials including polycarbonate for improving safety in railroad vehicles. Limited low impact testing was performed with 0.22 caliber ballistic testing. Proof testing of polycarbonate shields for laboratory protection was stressed by W. H. Ciolek, xe2x80x9cLaboratory Shielding for Projectiles,xe2x80x9d Proceedings of the American Institute of Chemical Engineers, National Meeting 1986, AlChE, New York, Pap 143c, who performed some limited impact testing on the material. The targets impacted were all 12 inch square without much detail given on support methods. The U.S. Naval Civil Engineering Laboratory was concerned with protecting building occupants from an external terrorist threat, such as an explosive blast, and developed a cable suspended polycarbonate window shield to absorb blast energy and shield against fragments from such explosions. More recently, workers in Germany have been investigating the use of polycarbonate for shielding personnel from machine tools. They have reported testing with small masses, (0.22-11) lb, striking a clamped 19.5 inch square target.
A successful containment shield should accomplish two goals: most importantly, it must stop the energetic fragment. In addition, it should stop the fragment with minimal movement of the shield. There will be significant transfer of energy from the fragment to the shield; if that energy moves the shield too far, then that movement could also cause damage to the surroundings the shield is intended to protect.
It is an object of this invention to provide an inexpensive modular shield that can withstand the impact from a piece thrown from a centrifuge.
It is another object of this invention to provide a shield formed of a standard sheet of polycarbonate held in place by conventional metal U-channel.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention may comprise a modular system for containing projectiles including a sheet of polycarbonate material held by a metal frame having a straight frame member corresponding to each straight edge of the sheet. Each frame member comprises a U-shaped shield channel covering and holding a straight edge of the sheet and an adjacent U-shaped clamp channel rigidly held against the shield channel. A flexible gasket separates each sheet edge from its respective shield channel; and each frame member is fastened to each adjacent frame member only by clamping means extending between adjacent clamp channels.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.